How do black holes emit light?

If there’s one thing everyone knows about black holes, it’s that they give off no light. These behemoths of time and space erase objects from history, swallowing everything in their paths. Yet a few minutes of research will tell you we can detect black holes through their radiation. In fact, black holes are supposedly some of the brightest objects in the universe. Welcome to the bizarre world of astrophysics.

First of all, your science teachers were telling the truth: black holes give off no light. As their namesake implies, these objects have grown so massive not even light can escape their gravity. If a rogue black hole flew between us and the stars, the only way to detect it would be through its gravity. Astrophysicists have shown, however, that most matter in the universe clumps together. Stars live in galaxies, and galaxies move in clusters. Black holes often live at the centers of galaxies or alongside stars throughout them.

An artist’s rendition of an accretion disc. Scientists have yet to understand why the black hole emits particle streams on polar axes.

When two stars come close to one another, they enter a gravitational dance. Both attracted by the other’s mass, they orbit each other, often stabilizing in what is called a binary system. This can happen with a black hole as well. Stars fortunate enough to avoid consumption can become locked in a dangerous ballet, zooming around the behemoth’s monstrous pull. Oftentimes, the black hole’s gravity siphons the outer layers of nearby stars, tugging the plasma and gas into a spiral of no return. This collection of matter is called a black hole’s accretion disc.

The final piece of the puzzle involves tidal forces. We might notice it, but every day we experience tidal forces. The moon’s gravitational pull varies with its distance to Earth, and parts of the Earth closest to the moon experience it more strongly than others (this explains the tides, though the details are a bit more complicated). Imagine now that the moon was millions of times more massive and the pull on the near side of Earth was millions of times more forceful than the pull on the far side. This is what happens to the gas in a black hole’s accretion disc. Massive tidal forces cause the layers to rub together, heating them to incredible temperatures and causing them to release huge amounts of high-energy light. While the black hole technically does not give off light, the disc often glows more brightly than the neighboring stars.

Black holes often siphon the outer layers of nearby stars. This feeds their accretion discs, which emit massive amounts of high-energy light.

The more massive the black hole, the larger the accretion disc and the more light it emits. QUASARS, for example, are some of the most luminous objects ever discovered. These supermassive black holes from the beginnings of the universe dwarf the stars and galaxies billions of lightyears closer to us. The centers of young galaxies, some emit one thousand times the energy of the Milky Way’s 200-400 billion stars. I will talk more about these in a future post.

So this is how the darkest objects in the universe are simultaneously the brightest in the night sky. To me, this is one of the most fascinating things about physics. Our intuition breaks down at extreme scales, where paradoxes seem to abound. There is still much we have yet to understand.We have evolved to understand the universe from one perspective—different scales seem to be alien worlds. That’s my take, at least. What’s your take on the paradox and its explanation? Share your thoughts in the comments below. As always, please share, like, comment, and subscribe if you enjoy this post. If you want to hear more about food, Pokémon, music, and science, go ahead and check me out on Twitter. As always, please share, like, comment, and subscribe if you like the post. Don’t forget to subscribe for more science/ramblings every Wednesday–it’s FREE!